Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material, without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy the requirements of customers, resulting in a large loss of profits. By utilizing pre-press color proofing time and money can be saved.
One such commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,268,708 is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying a sufficient amount of thermal energy to the dye donor material to form an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem (which includes a lathe bed scanning frame, a translation drive, a translation stage member, a printhead, and a vacuum imaging drum), and thermal print media and dye donor material exit transports.
The operation of the above image processing apparatus comprises metering a length of the thermal print media (in roll form) from the material assembly or carousel. The thermal print media is then measured and cut into sheet form of the required length, transported to the vacuum imaging drum, registered, wrapped around and secured onto the vacuum imaging drum. Next a length of dye donor material (in roll form) is also metered out of the material supply assembly or carousel, then measured and cut into sheet form of the required length. It is then transported to and wrapped around the vacuum imaging drum, such that it is superposed in the desired registration with respect to the thermal print media (which has already been secured to the vacuum imaging drum).
After the dye donor material is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and the dye donor material on the spinning vacuum imaging drum while it is rotated past the printhead that will expose the thermal print media. The translation drive then traverses the printhead and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the dye donor material is then removed from the vacuum imaging drum. This is done without disturbing the thermal print media that is beneath it. The dye donor material is then transported out of the image processing apparatus by the dye donor material exit transport. Additional dye donor materials are sequentially superposed with the thermal print media on the vacuum imaging drum, then imaged onto the thermal print media as previously mentioned, until the intended image is completed. The completed image on the thermal print media is then unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport.
The material supply assembly comprises a carousel assembly mounted for rotation about its horizontal axis on bearings at the upper ends of vertical supports. The carousel comprises a vertical circular plate having in this case six (but not limited to six) material support spindles. These support spindles are arranged to carry one roll of thermal print media, and four rolls of dye donor material to provide the four primary colors used in the writing process to form the intended image, and one roll as a spare or for a specialty color dye donor material (if so desired). Each spindle has a feeder assembly to withdraw the thermal print media or dye donor material from the spindles to be cut into a sheet form. The carousel is rotated about its axis into the desired position, so that the thermal print media or dye donor material (in roll form) can be withdrawn, measured, and cut into sheet form of the required length, and then transported to the vacuum imaging drum.
The scanning subsystem or write engine of the lathe bed scanning type comprises a mechanism that provides the mechanical actuators, for the vacuum imaging drum positioning and motion control to facilitate placement, loading onto, and removal of the thermal print media and the dye donor material from the vacuum imaging drum. The scanning subsystem or write engine provides the scanning function by retaining the thermal print media and dye donor material on the rotating vacuum imaging drum, which generates a once per revolution timing signal to the data path electronics as a clock signal; while the translation drive traverses the translation stage member and printhead axially along the vacuum imaging drum in a coordinated motion with the vacuum imaging drum rotating past the printhead. This is done with positional accuracy maintained, to allow precise control of the placement of each pixel, in order to produce the intended image on the thermal print media.
The lathe bed scanning frame provides the structure to support the vacuum imaging drum and its rotational drive. The translation drive with a translation stage member and printhead are supported by two translation bearing rods that are substantially straight along their longitudinal axis and are positioned parallel to the vacuum imaging drum and lead screw. Consequently, they are parallel to each other therein forming a plane, along with the vacuum imaging drum and lead screw. The translation bearing rods are, in turn, supported by outside walls of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine. The translation bearing rods are positioned and aligned therebetween, for permitting low friction movement of the translation stage member and the translation drive. The translation bearing rods are sufficiently rigid for this application, so as not to sag or distort between the mounting points at their ends. They are arranged to be as exactly parallel as is possible with the axis of the vacuum imaging drum. The front translation bearing rod is arranged to locate the axis of the printhead precisely on the axis of the vacuum imaging drum with the axis of the printhead located perpendicular, vertical, and horizontal to the axis of the vacuum imaging drum. The translation stage member front bearing is arranged to form an inverted "V" and provides only that constraint to the translation stage member. The translation stage member with the printhead mounted on the translation stage member, is held in place by only its own weight. The rear translation bearing rod locates the translation stage member with respect to rotation of the translation stage member about the axis of the front translation bearing rod. This is done so as to provide no over constraint of the translation stage member which might cause it to bind, chatter, or otherwise impart undesirable vibration or jitters to the translation drive or printhead during the writing process causing unacceptable artifacts in the intended image. This is accomplished by the rear bearing which engages the rear translation bearing rod only on a diametrically opposite side of the translation bearing rod on a line perpendicular to a line connecting the centerlines of the front and rear translation bearing rods.
The translation drive is for permitting relative movement of the printhead by synchronizing the motion of the printhead and stage assembly such that the required movement is made smoothly and evenly throughout each rotation of the drum. A clock signal generated by a drum encoder provides the necessary reference signal accurately indicating the position of the drum. This coordinated motion results in the printhead tracing out a helical pattern around the periphery of the drum. The above mentioned motion is accomplished by means of a DC servo motor and encoder which rotates a lead screw that is typically, aligned parallel with the axis of the vacuum imaging drum. The printhead is placed on the translation stage member in a "V" shaped groove, which is formed in the translation stage member, which is in precise positional relationship to the bearings for the front translation stage member supported by the front and rear translation bearing rods. The translation bearing rods are positioned parallel to the vacuum imaging drum, so that it automatically adopts the preferred orientation with respect to the surface of the vacuum imaging drum.
The printhead is selectively locatable with respect to the translation stage member, thus it is positioned with respect to the vacuum imaging drum surface. The distance between the printhead and the vacuum imaging drum surface is adjustable for focus. Extension springs provide the load against the adjustment screws for this focus adjustment. The angle of the printhead is also adjustable by rotating the cylindrical lens body. Here also, extension springs provide the load against the adjustment screw.
The translation stage member and printhead are attached to a rotatable lead screw (having a threaded shaft) by a drive nut and coupling. The coupling is arranged to accommodate misalignment of the drive nut and lead screw so that only rotational forces and forces parallel to the lead screw are imparted to the translation stage member by the lead screw and drive nut. The lead screw rests between two sides of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine, where it is supported by deep groove radial bearings. At the drive end the lead screw continues through the deep groove radial bearing, through a pair of spring retainers, that are separated and loaded by a compression spring to provide axial loading, and to a DC servo drive motor and encoder. The DC servo drive motor induces rotation to the lead screw moving the translation stage member and printhead along the threaded shaft as the lead screw is rotated. The lateral directional movement of the printhead is controlled by switching the direction of rotation of the DC servo drive motor and thus the lead screw.
The printhead includes a plurality of laser diodes which are coupled to the printhead by fiber optic cables which can be individually modulated to supply energy to selected areas of the thermal print media in accordance with an information signal. The printhead of the image processing apparatus includes a plurality of optical fibers coupled to the laser diodes at one end and the other end to a fiber optic array within the printhead. The printhead is movable relative to the longitudinal axis of the vacuum imaging drum. The dye is transferred to the thermal print media as the radiation, transferred from the laser diodes by the optical fibers to the printhead and thus to the dye donor material is converted to thermal energy in the dye donor material.
The vacuum imaging drum is cylindrical in shape and includes a hollowed-out interior portion. The vacuum imaging drum further includes a plurality of holes extending through its housing for permitting a vacuum to be applied from the interior of the vacuum imaging drum for supporting and maintaining the position of the thermal print media and dye donor material as the vacuum imaging drum rotates. The ends of the vacuum imaging drum are enclosed by cylindrical end plates. The cylindrical end plates are each provided with a centrally disposed spindle which extends outwardly through support bearings and are supported by the lathe bed scanning frame. One of the spindles is a drive end spindle that extends through the support bearing and is stepped down to receive a DC drive motor rotor which is held on by means of a nut. A DC motor stator is stationarily held by the lathe bed scanning frame member, encircling the armature to form a reversible, variable speed DC drive motor for the vacuum imaging drum. At the end of the spindle an encoder is mounted to provide the timing signals to the image processing apparatus. The opposite spindle is a vacuum spindle and is provided with a central vacuum opening, which is in alignment with a vacuum fitting with an external flange that is rigidly mounted to the lathe bed scanning frame. The vacuum fitting has an extension which extends within but is closely spaced from the vacuum spindle, thus forming a small clearance. With this configuration, a slight vacuum leak is provided between the outer diameter of the vacuum fitting and the inner diameter of the opening of the vacuum spindle. This assures that no contact exists between the vacuum fitting and the vacuum imaging drum which might impart uneven movement or jitters to the vacuum imaging drum during its rotation.
The opposite end of the vacuum fitting is connected to a high-volume vacuum blower which is capable of producing 50-60 inches of water (93.5-112.2 mm of mercury) at an air flow volume of 60-70 cfm (28.368-33.096 liters per second). This provides the vacuum to the vacuum imaging drum to support the various internal vacuum levels of the vacuum imaging drum required during the loading, scanning and unloading of the thermal print media and the dye donor materials to create the intended image. With no media loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 10-15 inches of water (18.7-28.05 mm of mercury). With just the thermal print media loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 20-25 inches of water (37.4-46.75 mm of mercury); this is the level required when a dye donor material is removed so that thermal print media does not move, otherwise color to color registration will not be maintained. With both the thermal print media and dye donor material completely loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 50-60 inches of water (93.5-112.2 mm of mercury) in this configuration.
The outer surface of the vacuum imaging drum is provided with an axially extending flat, which extends approximately 8 degrees of the vacuum imaging drum circumference. The vacuum imaging drum is also provided with a circumferential recess which extends circumferentially from one side of the axially extending flat, circumferentially around the vacuum imaging drum to the other side of the axially extending flat, and from approximately one inch (25.4 mm) from one end to approximately one inch (25.4 mm) from the other end of the vacuum imaging drum. The thermal print media when mounted on the vacuum imaging drum is seated in the circumferential recess and therefor the circumferential recess has a depth substantially equal to the thermal print media thickness seated therewithin, which is approximately 0.004 inches (0.102 mm) in thickness.
The purpose of the circumferential recess on the vacuum imaging drum surface is to eliminate any creases in the dye donor materials, as they are drawn down over the thermal print media during the loading of the dye donor materials. This assures that no folds or creases will be generated in the dye donor materials which could extend into the image area and seriously adversely affect the intended image. The circumferential recess also substantially eliminates the entrapment of air along the edge of the thermal print media, where it is difficult for the vacuum holes in the vacuum imaging drum surface to assure the removal of the entrapped air. Any residual air between the thermal print media and the dye donor material, can also adversely affect the intended image.
The purpose of the vacuum imaging drum axially extending flat is two-fold. First, it assures that the leading and trailing ends of the dye donor material are somewhat protected from the effect of air turbulence during the relatively high speed rotation that the vacuum imaging drum undergoes during the imaging process. Thus the air turbulence would have less tendency to lift the leading or trailing edges of the dye donor material. Second, the vacuum imaging drum axially extending flat also ensures that the leading and trailing ends of the dye donor material are recessed from the vacuum imaging drum periphery. This reduces the chance that the dye donor material can come in contact with other parts of the image processing apparatus, such as the printhead, causing a jam and possible loss of the intended image or worse, catastrophic damage to the image processing apparatus.
Further, the vacuum imaging drum axially extending flat acts to impart a bending force to the ends of the dye donor materials when they are held onto the vacuum imaging drum surface by vacuum from within the interior of the vacuum imaging drum. Consequently when the vacuum is turned off to that portion of the vacuum imaging drum, the end of the dye donor material will tend to lift from the surface of the vacuum imaging drum. Thus turning off of the vacuum eliminates the bending force on the dye donor material, and is used as an advantage in the removal of the dye donor material from the vacuum imaging drum.
The task of loading and unloading the dye donor materials onto and off from the vacuum imaging drum, requires precise positioning of thermal print media and the dye donor materials. The lead edge positioning of dye donor material must be accurately controlled during this process. Existing image processing apparatus designs, such as that disclosed in the above commonly assigned U.S. patent, employs a multi-chambered vacuum imaging drum for such lead-edge control. One appropriately controlled chamber applies vacuum that holds the lead edge of the dye donor material. Another chamber, separately valved, controls vacuum that holds the trail edge of the thermal print media, to the vacuum imaging drum. With this arrangement, loading a sheet of thermal print media and dye donor material requires that the image processing apparatus feed the lead edge of the thermal print media and dye donor material into position just past the vacuum ports controlled by the respective valved chamber. Then vacuum is applied, gripping the lead edge of the dye donor material against the vacuum imaging drum surface.
Unloading the dye donor material or the thermal print media (to discard the used dye donor material or to deliver the finished thermal print media to an output tray) requires the removal of vacuum from these same chambers so that an edge of the thermal print media or the dye donor material are freed and project out from the surface of the vacuum imaging drum. The image processing apparatus then positions an articulating skive into the path of the free edge to lift the edge further and to feed the dye donor material, to a waste bin or an output tray.
The sheet material exit transports include a dye donor material waste exit and the imaged thermal print media sheet material exit. The dye donor material exit transport comprises a waste dye donor material stripper blade disposed adjacent the upper surface of the vacuum imaging drum. In an unload position, the stripper blade is in contact with the waste dye donor material on the vacuum imaging drum surface. When not in operation, the stripper blade is moved up and away from the surface of the vacuum imaging drum. A driven waste dye donor material transport belt is arranged horizontally to carry the waste dye donor material, which is removed by the stripper blade from the surface of the vacuum imaging drum to an exit formed in the exterior of the image processing apparatus. A waste bin for the waste dye donor material is separate from the image processing apparatus. The imaged thermal print media sheet material exit transport comprises a movable thermal print media sheet material stripper blade that is disposed adjacent to the upper surface of the vacuum imaging drum. In the unload position, the stripper blade is in contact with the imaged thermal print media on the vacuum imaging drum surface. In the inoperative position, it is moved up and away from the surface of the vacuum imaging drum. A driven thermal print media sheet material transport belt is arranged horizontally to carry the imaged thermal print media removed by the stripper blade from the surface of the vacuum imaging drum. It then delivers the imaged thermal print media with the intended image formed thereon to an exit tray in the exterior of the image processing apparatus.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. The front and rear bearing translation rods must be substantially straight to provide a structure that allows the desired degree of accuracy. Among the various methods used to straighten these rods are magnets that are mounted on the image processing apparatus, as disclosed in copending patent application U.S. Ser. No. 08/667,775 filed Jun. 21, 1996; entitled: AN APPARATUS FOR MAINTAINING THE POSITIONAL RELATIONSHIP OF A PRINT HEAD. However, using magnetic means to straighten these bearing rods in place can also affect printhead travel, introducing undesirable changes in friction as the printhead translation assembly passes over magnet positions. While the printhead translation assembly is maintained in position against the bearing rods substantially by its own weight, the addition of magnetic attraction toward the underlying bearing rod increases the torque required to move the printhead along its path of travel.
Conventional solutions for holding the printhead translation assembly against the front and rear bearing rods have unwanted side effects. For example, wheels or leaf spring arrangements require the addition of complex mechanical parts. Linear bearings (such as a ball slide bearing) would be prohibitively expensive, considering the length of the printhead travel path (typically more than 12 inches in imaging devices).
While normally the attractive force of permanent magnets is utilized, magnets have also been used in different types of devices to take advantage of repulsive force. Some typical uses include the following:
U.S. Pat. No. 5,017,819 to Patt, et al. which discloses the use of the linear spring force characteristics of magnets in any direction relative to an orthogonal coordinate system, with primary application to motor use. This patent concerns itself with methods for achieving a linear force constant for a long movement of the magnetic spring and for controlled oscillation (such as is needed with Stirling refrigeration motors).
U.S. Pat. No. 5,148,066 to Beale, et al. which discloses the use of magnetic springs for applying centering bias on a piston in a linear generator or motor.
U.S. Pat. No. 5,038,063 to Graber, et al. which discloses the use of a permanent magnet in combination with an electromagnet to form a magnetic spring; and
U.S. Pat. No. 4,863,240 to Nakajima, et al. which discloses a design of a magnetic spring to constrain an objective lens in a neutral position.
While these patents and other known applications show the use of electromagnetic devices and permanent magnets to provide repulsive force, none of the patents cited above discloses or suggests positioning magnets so as to take advantage of magnetic repulsion to minimize friction for a table moving linearly along a support structure.
Therefore, the need exists for a simple method for maintaining the printhead translation assembly in position throughout its travel path, while, at the same time, minimizing the added friction caused by magnets used to straighten the translation bearing rods that form the support structure.
A further drawback of conventional arrangements is that a complex combination of support components is required to hold the printhead so that it is perfectly aligned orthogonally with respect to the vacuum imaging drum surface; and so that the printhead has the correct focus, which is determined by its distance from the drum surface (held constant throughout the printhead's travel path across the vacuum imaging drum). Also, the printhead must be adjusted for the proper rotational angle about its focus axis and must be held in position so as to maintain this angle throughout its travel path across the vacuum imaging drum. (This angle provides the intended swath width).
The focus and rotational angle must be initially adjusted in manufacture and may need to be adjusted once the equipment is installed and periodically during its lifetime. As described, the above-mentioned printhead translation stage uses spring-loaded mechanisms to achieve and maintain these critical adjustments. The existing system requires that a number of precision components be assembled on the translation stage, which adds considerable expense and complexity to the translation stage and makes the job of printhead adjustment or replacement an expensive, time-consuming operation.
To meet the requirements for adjustability and for maintaining these critical adjustments throughout its travel, the present invention further uses magnets to hold the printhead on the translation stage subassembly. This solution eliminates parts and cost, while meeting the requirements stated above.
Magnets however, can exhibit relatively poor bearing characteristics. As is well known in the art, the magnet surface using known magnetic materials is not designed to withstand frictional forces. This presents a problem with the use of magnets for this application. The magnets must be selected to have sufficient holding power to hold the printhead securely. But, at the same time, the magnets used to hold the printhead securely in place must also allow periodic adjustment. This requires that the magnet surface allow movement of the printhead against it, during adjustment, while withstanding any damage from frictional forces. Known approaches for protecting the magnet surface against sliding damage as documented by Lester R. Moskowitz, Permanent Magnet Design and Application Handbook, Robert E. Krieger Publishing Company, 1976 (page 81ff.), are concerned with increasing, rather than decreasing the frictional characteristics of the magnet surface.
Magnets may be selected from different types; for the preferred embodiment of this invention, coated magnets have been chosen. Coating of magnets is widely used to provide a surface that resists oxidation or corrosion in numerous applications, including use with dental appliances and veterinary instruments (where the magnetic material must be protected from corrosion, such as is disclosed in U.S. Pat. No. 4,857,873 where a dental device must be protected from saliva). But such coatings bonded to the magnet are optimized to prevent oxidation and corrosion and not to provide a bearing surface capable of withstanding sliding forces across its surface during adjustment, without damage.
Caps for magnets are used in various applications where the magnet must be protected, such as is disclosed for fabricating a sensor component in U.S. Pat. No. 5,213,251, in which a cap is applied to a magnetized piece, then the assembled unit is case-hardened for wear-resistance. This patent discloses protection of the magnet from mechanical wear, but not due to sliding forces.
It is important to note that the magnets used to hold the printhead in this application are fabricated and mounted with precision tolerance so that they align the printhead so that its focal axis is normal to the cylindrical surface of the vacuum imaging drum. Softer magnets, if selected for this application, could become worn as these magnets are subjected to wear. This could alter the precision alignment that these magnets are designed to provide.
Magnets are widely used in the art for their holding properties in various precision applications. However, the specific problem outlined above has not been addressed, since the existing applications have not applied magnets for holding other components where periodic adjustment requires that the magnet present an effective, durable bearing surface, having a low coefficient of friction that allows controlled incremental movement for precise adjustment.